Pro

Good navigation

Keyboard shortcuts (and toolbar buttons) to navigate to next (previous) difference, side panel shows a map of the files with changed lines and allows to jump to a given place.

Pro

File edition

You can quickly copy changed lines (or files in folder comparison) in both directions with keyboard shortcuts. You can edit the files as well, with syntax highlighting of some languages.

Pro

Good shell integration

Select two files and compare them. Alternatively, select one file, navigate elsewhere, select the other file to compare.Also supports drag'n'drop of files / folders from Explorer.History of past comparisons.

Cons

Con

Development is stopped

No update for years; last release was made in 2013.

There's a fork, named WinMerge2011: https://bitbucket.org/jtuc/winmerge2011/Some years ago it had regressions from the official version but now (2015) it seems at least on par with the historical version, and it has a 64-bit version.

Con

Windows only

It's only available for Windows. No Mac or Linux versions available.

Con

No 3-way merge

Cannot merge 3 files, can do only comparisons by pairs.Makes it unsuitable for merging operations, still useful to compare two versions in the history.